At this point it may be useful to reflect upon the popular conception of correctness in language since, if it is to deal adequately with linguistic reality, it needs refining. How can we judge the ‘correctness’ of some form of expression without taking into account the circumstances in which the expression is being used? For instance, is it ‘correct’ to say smack in the middle, or should I say right in the middle? The informal expression would of course be inappropriate to a situation requiring formality. But informal language is equally systematic, and to equate correctness with formality would be to ignore this. It is obviously necessary to allow that language varies according to circumstances, and what is appropriate in one case may be inappropriate in another. The kinds of factors that change from one situation to another are: first, that the social roles of those who are addressing each other are various; second, the kind of social activity that is going on may be different; and third, the medium of communication can change, for instance, from spoken to written medium. So far we have found two kinds of language variation: there are varieties used by different groups of speakers—dialects; and there are varieties used for different social purposes— registers. (The reader should be wary of the latter term since it is also used in other senses and may be a source of misunderstanding; see the glossary.) In general, a single speaker commands one dialect but has a range of registers at his disposal.